pH
Chapter 19.3
Vocabulary Word
• hydronium ion: H3O+
• hydroxide ion: OH-
Ionization of Water
• When water ionizes it is acting as both an
acid and a base in the same reaction
H2O  H+ + OHKeq =
[H+] [OH-]
[H2O]
Ionization of Water
Keq [H2O] = Kw = [H+] [OH-]
• At 298K experimentally the Keq of water is
1.0 x 10 -14
• Kw = 1.0 x 10 -14
• [H+] = [OH-] = 1.0 x 10 -7
Using Kw to Calculate [H+] or
[OH-]
• If [H+] = 1.0 x 10 -5, what is [OH-]?
Kw = [H+] [OH-]
1.0 x 10 -14 = 1.0 x 10 -5 [OH-]
1.0 x 10 -14 = [OH-]
1.0 x 10 -5
[OH-] = 1.0 x 10 -9
Acid or Base
1.0 x 10 -5 > 1.0 x 10 -9
[H+] > [OH-]
Therefore the solution is an acid.
pH Scale
• Since the concentrations of H+ and OH- are
so small, chemists adapted a simpler way to
express them on a log scale, pH.
• pH = - log [H+]
Vocabulary Word
• pH = - log [H+], values range from 0 to 14.
Acids have a pH less than 7. Bases have pH
greater than 7.
Log Scales
• Since pH is measured on a log scale, each
unit is a 10 fold change in [H+].
• A difference in pH of 2 is a 100 fold change
in [H+]
• A difference in pH of 5 units is a 100,000
change in [H+]
Calculating Acidity
• pH can also be calculated from the [H+]
• If [H+] = 1.0 x 10 -2
• pH = - log 1.0 x 10 -2 = 2.0
Calculating Basicity
• Sometimes chemists calculate basicity
instead
• pOH is a measure of basicity
• pOH = - log [OH-]
Vocabulary Word
• pOH: - log [OH-], values range from 0 to
14. Acids have a pOH of greater than 7 and
bases have a pOH of less than 7.
Calculating pH or pOH
• pH + pOH = 14
• If [OH-] = 4.0 x 10 -3, calculate the pH and
the pOH.
• given [OH-], so pOH = - log 4.0 x 10 -3
• pOH = 2.4
• pH = 14 - 2.4 = 11.6
Calculating [H+] or [OH-]
• If the pH of a solution is 3.5, calculate [H+]
and [OH-]
• given pH, so 3.5 = - log [H+]
• [H+] = antilog -3.5
• [H+] = 3.2 x 10 -4
Calculating [H+] or [OH-]
•
•
•
•
•
If pH = 3.5, pOH = 14 - 3.5 = 10.5
pOH = - log [OH-]
10.5 = - log [OH-]
antilog -10.5 = [OH-]
[OH-] = 3.16 x 10 -11
Measuring pH
• Litmus can distinguish acids from bases
– acids turn litmus red
– bases turn litmus blue
• A quantitative value of pH can be measured
using:
– indicator paper (pH paper)
– a pH meter